Upgrade Java to 8

[ys8]

The project already depends on Guava 26.0-jre, which reuqires Java 8. Without this change, the project doesn't even compile in IntelliJ.

[rafael]

@ys8 could you open this PR upstream?

[ys8]

Sure. I have other fixes following this PR including

vitessio#4366

and another fix to allow vtgate discovery through consul-template. Should I do all those work in upstream? @demmer asked me to push those fixes to this repo. I'm a newbie to this process, so would appreciate your guidance...

[demmer]

Sorry I didn’t clarify... what we usually do is open a pr here referencing the upstream master.

Then periodically we sync upstream to this master branch for internal deploys.

Many of us do this by having two remotes in our local git environment.

[rafael]

@ys8 - That is correct. We try to keep in sync our fork with upstream, so unless we have some specific reasons we do all our work there.

We have diverged in the past when we needed some changes in an urgent way and we couldn't get them fast enough in upstream. However, this was a temporary and eventually it converged.

[ys8]

With @rafael 's help, finally opened a PR hopefully in a correct way.

vitessio#4405

For some reasons, it didn't let me edit the reviewers to include @demmer.